1. Field of the Invention
The present invention relates to vehicle parking brakes for vehicles equipped with hydraulic brake systems, and especially heavy vehicles, such as trucks and the like.
2. Description of the Prior Art
In the past, most vehicles, such as automotive vehicles, have been equipped with parking brake systems. However, conventional parking brakes are invariably mechanically operated devices. A typical automotive vehicle parking brake employs a lever in the vehicle cab or other operator area which draws on a cable to pull conventional disk brake pads or drum brake shoes into frictional engagement with a disk or drum associated with each of the rear wheels of a vehicle or with a special dedicated parking brake located at the vehicle transmission. The lever is manually operated, either by hand or by foot. In this mechanical system a pawl is disposed to allow actuation of the lever when physical force is applied to engage the brake, but the pawl engages a rack to prevent reverse movement. The force on the brake pads or shoes is directly proportional to the physical force applied to the lever. A latch must be manually withdrawn to release the pawl when it is desired to disengage the brake.
In alternative systems, hydraulic pressure is used. Again, a hand or foot lever is physically operated to exert hydraulic pressure on the brake pads or shoes to lock the parking brake. Once the hydraulic pressure is applied, a hydraulic valve is closed to lock the pressure onto the service brakes. The valve may be manually actuated, or it may be actuated remotely through a switch which controls an electric solenoid to close the valve.
The conventional parking brake systems employed with vehicles have been deficient in operation in several respects. This particularly true with respect to heavy vehicles, such as beverage trucks. Because of the large weight involved, the strength required to engage the manually actuable mechanical parking brake levers is quite large. Very frequently through operator inattention or lack of the requisite manual strength, the mechanical parking brakes conventionally employed are not completely engaged because insufficient force is applied to the brake lever. As a result, there are presently numerous instances of runaway vehicles that result from parking brake failure. These occurrences are especially prevalent where beverage trucks, or other heavy vehicles are parked on inclines. The problem is particularly acute with beverage trucks, since these vehicles are usually utilized to make frequent stops and the parking brakes employed therewith are very frequently engaged and disengaged throughout the course of a delivery run.
Another disadvantage of the conventional mechanical parking brake systems is that they are rated and specified for use with vehicles, the weights of which are calculated in an unloaded condition by the responsible regulatory authorities. During actual use, however, the loaded weight of the vehicle is double or even more than twice as great as its empty weight. Also, regulations typically require parking brakes to hold a vehicle on a 20% grade. In actual use, however, vehicles are frequently required to traverse grades of 30 to 35%, particularly in mountainous or hilly areas. As a consequence, although, a vehicle weight any dictate the requirement for a mechanical parking brake rated for example, as requiring 90 lbs. of input pressure for actuation, such a brake has inadequate holding power in actual use.